1. Field of the Invention
The present invention relates to an electro-optical device, typically an EL (electroluminescence) display device formed by a semiconductor element (an element using a semiconductor thin film) made on a substrate, and to electronic equipment (an electronic device) having the electro-optical device as a display (also referred to as a display portion).
2. Description of the Related Art
Techniques of forming a TFT on a substrate have been widely progressing in recent years, and developments of applications to an active matrix type display device are advancing. In particular, a TFT using a. polysilicon film has a higher electric field effect mobility (also referred to as mobility) than a TFT using a conventional amorous silicon film, and high speed operation is therefore possible. As a result, it becomes possible to perform pixel control, conventionally performed by a driver circuit external to the substrate, by a driver circuit formed on the same substrate as the pixel.
This type of active matrix display device has been in the spotlight because of the many advantage which can be obtained by incorporating various circuits and elements on the same substrate in this type of active matrix display device, such as reduced manufacturing cost, small size, increased yield, and higher throughput.
An EL layer (light emitting layer) is made to emit light in an active matrix EL display device by disposing a switching element formed from a TFT to each pixel, and by driving a driver element that performs current control by the switching element. For example, there are EL display devices disclosed in U.S. Pat. No. 5,684,365 (Japanese Patent Application Laid-Open No. Hei 8-234683) and Japanese Patent Application Laid-Open No. Hei 10-189252.
Degradation of EL materials due to moisture has been a problem in these EL display devices. Specifically organic EL materials degrade not only by moisture but also by oxygen. Accordingly EL elements are generally shielded from moisture, etc., by sealing the EL elements as disclosed in Japanese Patent Application Laid-Open No. Hei 8-78159.
However, the problem that the EL layer has is not limited to moisture. The EL layer includes alkaline metals such as sodium (Na) in itself, and a serious trouble can be caused to the operation of TFTs when the alkali metals are diffused into the TFTs. Further, degradation due to storage of heat is also a problem because an EL layer is weak to heat. Note that alkaline metals are referred to ‘alkaline metals’ to include alkali earth metals through the Specification.